1. Field of the Invention
This invention relates to a coating composition which provides under irradiation with ultraviolet rays a crosslinked and cured coating film excellent in the abrasion resistance, flexibility, heat resistance, solvent resistance, adhesion to the substrate and weatherability, and also to a process for preparing synthetic resin shaped articles excellent in the abrasion resistance and weatherability by using this coating composition.
2. Description of the Prior Art
Synthetic resin shaped articles prepared from polymethyl methacrylate resins, polycarbonate resins, polystyrene resins and AS resins have a light weight and are excellent in their impact resistance, and these synthetic resins are cheap and can easily be molded. Because of these various advantages, these synthetic resin shaped articles have been used in various fields and new uses of these synthetic resin shaped articles have been developed in various fields. However, these synthetic resin shaped articles are poor in the abrasion resistance, and therefore, the surfaces are readily damaged by contact with a hard article or abrasion or scratching. Damage caused on the surfaces of these shaped articles lower the commercial value thereof drastically and shorten the service lives of these shaped articles. Therefore, it has eagerly been desired to improve the abrasion resistance of the surface in these synthetic resin shaped articles. Shaped articles having an improved abrasion resistance, which are prepared by coating the above-mentioned synthetic resin shaped article with a coating material, are often used outdoors. For example, they are used as windshields of air planes, outdoor signboards, traffic control signs and automobile parts, and therefore, a high weatherability is also eagerly desired in these shaped articles.
Various methods have heretofore been proposed as means for eliminating these defects of the above-mentioned synthetic resin shaped articles. For example, there can be mentioned a method in which a coating material comprised of a resin composition comprising a silicone or melamine type resin is coated on the surface of a synthetic resin shaped article and then, the coating material is heated to be thereby condensed to yield a crosslinked coating film having an improved abrasion resistance on the synthetic resin shaped article, and a method in which a polyfunctional monomer containing at least one polymerizable group in the molecule is coated on the surface of a synthetic resin shaped article and then, the coated shaped article is irradiated with active energy rays to form a crosslinked and cured coating film on the surface of the synthetic resin shaped article, whereby a synthetic resin shaped article having an excellent abrasion resistance is obtained (see U.S. Pat. Nos. 3,518,341 and 4,041,120 and Japanese Laid-Open Patent Application No. 102,936/78, No. 104,638/78 and No. 97,633/79).
Indeed, the abrasion resistance of the surface of synthetic resin shaped articles can be improved by these methods. However, these methods are not completely satisfactory in some points. For example, the products obtained according to the former method are poor in durability to thermal shock and hot water, and in chemical resistance. Furthermore, the products obtained according to the latter method are poor in weatherability though they are satisfactory in durability and chemical resistance.
Moreover, in the case where a synthetic resin shaped article on which an abrasion-resistant surface coating film is to be formed has a poor weatherability, for example, when the shaped article is comprised of a polycarbonate resin or the like, even if the weatherability of the abrasion-resistance surface coating film per se is excellent, the shaped article is deteriorated by active energy rays, such as ultraviolet rays, permeating through the surface coating film, with the result that strong yellowing is caused in the shaped article, cracks are formed on the cured surface coating film and in extreme cases, the surface coating film is undesirably separated from the shaped article.
Under such background, we did research with a view to developing a synthetic resin shaped article excellent in not only abrasion resistance but also weatherability, and we have now found that when a coating composition comprising at specific ratios a monomer mixture comprising a specific polyfunctional methacrylate or acrylate and a monofunctional or bifunctional methacrylate or acrylate in a specific proportion, optionally with a monomer having a specific structure, a homopolymer or copolymer of an alkyl methacrylate or acrylate, an ultraviolet absorber, and a photosensitizer is coated on a synthetic resin shaped article and is then cured, a synthetic resin shaped article having excellent abrasion resistance and weatherability can be advantageously obtained on an industrial scale.